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What types of ion sources are commonly used in liquid mass spectrometers?


Liquid chromatography mass spectrometry (LC/MS or LC/MS/MS), referred to as liquid-mass spectrometry, commonly used ion sources from the broad classification, the main three ionization modes are atmospheric pressure ion source (hereinafter referred to as API), matrix-assisted laser resolved ionization source (hereinafter referred to as MALDI), and fast atom bombardment source (hereinafter referred to as FAB).

1. Atmospheric Pressure Ion Sources (API)

Including atmospheric pressure electrospray ionization (ESI), atmospheric pressure chemical ionization APCI, atmospheric pressure photoionization APPI

Atmospheric pressure electrospray ionization (ESI)

In ESI, the formation of ions is the measured molecules are ejected during the continuous contraction of the charged droplets, i.e., the ionization is done in the liquid state. The sample solution separated by liquid chromatography flows into the ion source. Vaporized under the N2 flow into the strong electric field region, the strong electric field formed by the Coulomb force to make small droplets of sample ionization, with the help of countercurrent heating N2 molecular ion particles on the surface of the liquid further evaporation, so that the molecular ions repel each other to form a tiny molecular ion particles as shown in the figure. These ions may be singly or multiply charged, depending on the volume and number of acidic or basic groups in the resulting positively and negatively charged molecules. The formation of multicharged ion peaks enables even quadrupole filter mass spectrometers with a mass range of 3000u to detect the exact molecular weight of biomolecules.

Advantages of ESI

Highly charged ions can be generated without fragmentation, which reduces the mass-to-charge ratio to a level that can be detected by a variety of different types of mass analyzers. By detecting the charged state, the true molecular weight of the ion can be calculated. At the same time, resolving the isotopic peaks of the molecular ion also determines the number of charges and the molecular weight, since the difference in mass-to-charge ratio between the isotopic peaks corresponds to the number of charges. The advantage is that it can be easily coupled with separation techniques.

Disadvantages of ESI

The main disadvantage of ESI is that it can only accept very small liquid flow rates (1-10 μl/min), a disadvantage that has been overcome by the development of the Ion Spray Interface (ISP) in 1987 (the ISP is an electrospray interface with the aid of pneumatics, which accommodates higher flow rates).

Atmospheric Pressure Chemical Ionization (APCI)

APCI technology is different from the traditional chemical ionization interface, it does not use a reaction gas such as methane, but with the corona discharge to start a series of gas-phase reaction to complete the ionization process, the principle, it can also be called discharge ionization or plasma ionization. The sample solution from the liquid chromatography enters a capillary tube with an atomizing gas sleeve, is atomized by a stream of nitrogen gas, and is vaporized when it passes through a heated tube. At the end of the heated tube a corona discharge is performed, the solvent molecules are ionized and act as a reaction gas, colliding with the sample gaseous molecules. After a complex reaction process, the sample molecules generate quasi-molecular ions, which can also generate quasi-molecular ions in a negative ion mode, and are mainly applied to compounds with a strong electron affinity. The excimer ions of the sample molecules enter the mass spectrometer through the screening slit.

Atmospheric Pressure Photoionization (APPI)

APPI is a process in which the analyte is ionized in the gas phase by absorbing electrons at a specific energy level (10 or 10.6 eV) emitted from the vacuum-ultraviolet and then emitting electrons, APPI is less used. APPI is to ionize the substance to be measured directly and is more suitable for the analysis of non-polar or weakly polar compounds.

Advantages of APCI & APPI

Suitable for ionization of low-polar compounds; wide dynamic range (4-5 orders of magnitude); mass-sensitive, can tolerate high buffer concentrations

Disadvantages of APCI & APPI

Low thermal stability of compounds (up to 130-150 °C), volatile, requires dopant

2. Matrix-assisted laser resolving ionization source (MALDI)

MALDI is a mass spectrometry ionization method that directly vaporizes and ionizes nonvolatile samples, but the ionization mechanism is unclear, and there are two possibilities: the ions are already formed in the solid state and are simply released when the laser irradiates; or they are generated by a laser-induced ion-molecule reaction. However, the mechanism of ionization is not clear, and there are two possibilities: the ions are formed in the solid state and simply released when the laser is irradiated; or they are produced by laser-induced ion-molecule reactions.

Advantages of MALDI

Can ionize some difficult to ionize samples (especially biological macromolecules), complete ionization products, and no obvious debris; single-charged molecular ions account for the majority of peaks, mass spectrometry is simpler, suitable for multi-component samples; wide range of application, can tolerate a certain degree of salt and buffer; the requirements of the sample treatment is not strict, and can even directly analyze the biological samples that have not been handled, thus simplifying the cumbersome sample preparation process; high sensitivity. process; high sensitivity.

Disadvantages of MALDI

However, it is less used in the qualitative and quantitative analysis of organic small molecules and chemical components of tobacco smoke.

3. Fast atom bombardment (FAB) source

The method of ionizing organic compounds (target surface) coated on the surface of a metal with glycerol (substrate) by striking them with accelerated neutral atoms (fast atoms) is called fast atom bombardment (FAB). When electrons bombard a neutral gas (argon or helium) at an air pressure of about 100 Pa, the resulting noble gas ions are focused and accelerated to hit the target surface, resulting in the ionization of the analyte, which is called ion bombardment. On this basis, the argon ions are reduced to neutral atoms, and then the accelerated neutral atoms hit the target surface, which is called fast atom bombardment. Analytes by the impact of neutral atoms to obtain enough kinetic energy to ions or neutral molecules in the form of the target surface to escape into the gas phase. The resulting ions are generally excimer ions.

Advantages of FAB

It has unique strengths for the analysis of compounds that are thermally unstable and difficult to vaporize. In particular, its effectiveness for peptide and protein analysis was unmatched by other interfaces prior to the advent of electrospray interfaces.FAB has a large number of reports and examples of successful protein analysis in peptide and protein analysis, demonstrating strong utility in this field.

Disadvantages of FAB

Can only work at low flow rates (< 5 μl/min), which severely limits the separation of liquid phase columns. The 1~5% glycerol contained in the mobile phase causes the ion source to become dirty very quickly. The liquid passes through the quartz capillary tube easily causing clogging. In addition, due to its special sampling method, a big problem with FAB is the interference of co-existing substances in the mixture sample, which often inhibit the ionization of the analyte, resulting in reduced sensitivity or even no signal generation at all.